The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

De Montessus' earthquake map divides the grand divisions of the earth into numerous sub-divisions, too numerous, indeed, for even brief description in a work of this kind. From the map he thus laboriously prepared De Montessus drew the following general conclusions:

1. The parts of the earth that are most apt to be shaken by earthquakes are those which possess the greatest differences of relief between their highlands and lowlands, and that in such regions the most pronounced earthquakes are found on the steepest slopes.

2. Earthquakes are most common along those parts of the crust that are thrown up in huge wrinkles, or mountain ranges, whether these masses be above the level of the sea or are covered by it.

Fig. 55. Davison's Earthquake Map of Japan

3. Earthquakes are more common in mountainous districts than in plains. But not all mountains are characterized by earthquakes nor are all plains free from them. Sometimes the plain at the base of the mountain appears to be especially liable to shocks, probably by reason of slips along faults at these points.

The great mountain ranges of the world are generally characterized by unequal slopes, the long gentle slope facing the interior of the continents, and the short, abrupt slopes being turned towards the coast. Now, Montessus points out that volcanoes are the most frequent on the short, abrupt slopes. In some cases, however, where the long slopes are the roughest, it is these slopes that are most frequently shaken.

The beds of the ocean that lie along rapidly descending lines, especially when they lie on the borders of large mountain ranges, are especially liable to earthquakes.

Dr. Charles Davison has made a map of the earthquakes of Japan in which he had adopted the plan of representing the origin or centres of earthquakes by a series of contour lines like those employed on topographical maps. The advantage of this type of map over that employed by Mallet is just this: Davison's earthquake map of Japan in which the active volcanoes are marked by dots, and the earthquakes by contour lines surrounding the points of origin, discloses the interesting fact that here the positions of the volcanoes and the earthquake centres coincide, since the mountainous districts where the active volcanoes are numerous are singularly free from earthquakes. This can be seen from an inspection of [Fig. 55].